The retinoblastoma tumor suppressor protein (pRB) is a potent inhibitor of mammalian cell growth and the functional inactivation of pRB is widely presumed to be essential for progression of the cell cycle from G 1 phase. In this work, the generality of pRB-based cell cycle control in mammalian cells was addressed by conditionally expressing pRB in cytokine-dependent hematopoietic cells. We show herein that these cells are able to progress through the cell cycle in response to cytokine despite the continued presence of supraphysiological amounts of wild-type pRB or phosphorylation-resistant pRB mutants. However, their growth was strongly blocked by ectopic expression of the pRB-related pocket protein, p130. This growth inhibition required the E2F-binding pocket domain but not the cyclin-binding domain of p130. Furthermore, increased amounts of the p130-controlled E2F, termed E2F-4, potentiated the mitogenic response of the cells to cytokine and the constitutive overexpression of E2F-4 rendered the cells cytokine-independent. Our results indicate the existence of a non-pRB-based cell cycle whose operation depends primarily on the interplay between p130 and E2F-4 in certain hematopoietic cells.

The Elongin complex stimulates the rate of transcription elongation by RNA polymerase II by suppressing the transient pausing of the polymerase at many sites along the DNA template. Elongin is composed of a transcriptionally active A subunit and two small regulatory B and C subunits, the latter of which bind stably to each other to form a binary complex that interacts with Elongin A and strongly induces its transcriptional activity. To further understand the roles of Elongin in transcriptional regulation, we attempted to identify Elongin-related proteins. Here, we report on the cloning, expression, and characterization of human Elongin A2, a novel transcription elongation factor that exhibited 47% identity and 61% similarity to Elongin A. Biochemical studies have shown that Elongin A2 stimulates the rate of transcription elongation by RNA polymerase II and is capable of forming a stable complex with Elongin BC. However, in contrast to Elongin A, its transcriptional activity is not activated by Elongin BC. Northern blot analysis revealed that Elongin A2 mRNA was specifically expressed in the testis, suggesting that Elongin A2 may regulate the transcription of testis-specific genes.Eukaryotic messenger RNA synthesis is a complex biochemical process controlled by the concerted action of a set of general transcription factors that regulate the activity of RNA polymerase II during the initiation and elongation stages of transcription. At least six general transcription initiation factors (TFIIA, TFIIB, TFIID, TFIIE, TFIIF, and TFIIH) 1 have been identified in eukaryotic cells and found to promote selective binding of RNA polymerase II to promoters and to support a basal level of transcription (1). In addition to the general initiation factors, six general elongation factors (SII, P-TEF-b, TFIIF, Elongin, eleven-nineteen lysine-rich leukemia protein, and the Cockayne syndrome complementation group B protein) have been defined biochemically and found to increase the efficiency of transcription elongation by RNA polymerase II (2-4). Of the general elongation factors, SII and P-TEFb prevent RNA polymerase II from prematurely arresting transcription. All of the rest of the elongation factors, TFIIF, Elongin, eleven-nineteen lysine-rich leukemia protein, and Cockayne syndrome complementation group B protein, act to increase the overall rate of RNA chain elongation by RNA polymerase II by suppressing the transient pausing of polymerase at many sites along the DNA template.Elongin was initially identified as a heterotrimer composed of A, B, and C subunits of ϳ770, 118, and 112 amino acids, respectively (5-8). Elongin A is the transcriptionally active component of the Elongin complex, whereas Elongins B and C are positive regulatory subunits. Biochemical studies have shown that Elongins B and C form a stable Elongin BC complex that binds to Elongin A and strongly induces its transcriptional activity (8, 9). Elongin C functions as the inducing ligand and activates transcription through interaction with a short conserved motif ...

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Elongin A is the transcriptionally active subunit of the Elongin complex, which strongly increases the rate of elongation by RNA polymerase II by suppressing the transient pausing of the polymerase at many sites within transcription units. In the present study, we obtained the cDNA sequence of the mouse Elongin A gene (Tceb3) and characterized its genomic structure. The deduced 773-amino acid sequence of mouse Elongin A shows 91% and 81% identity with rat and human Elongin A, respectively. The Elongin A gene was mapped to mouse chromosome 4D3 by fluorescence in situ hybridization.